The invention disclosed herein relates to punching holes in workpieces with minimal or no distortion. The invention relates particularly to punching cross holes in relatively thin cross sections and/or relatively soft workpieces.
A cross hole, i.e., a hole which has its axis perpendicular to the major surfaces of the workpiece in which it is formed, may be made by drilling or punching. Drilling produces a hole with minimal distortion; that is, it produces a relatively straight hole and with minimal or no distortion of the workpiece, but drilling is a relatively slow and expensive operation. Where a burr-free hole is desired, it is common practice to drill part way through the workpiece from one side, and finish the hole by drilling from the other side, which makes the drilling operation even slower and more expensive.
A punched hole may be made much faster, and primarily for that reason is less expensive than a drilled hole. However, punching produces a hole with more distortion than a drilled hole, and also frequently distorts the workpiece. A punched hole may be made using a punch having the nominal size of the hole, and a die, which supports the workpiece being punched, having a slightly larger diameter hole than the diameter of the punch to permit removal of the punched-out material. For thinner metal workpieces, the die hole is just slightly larger than the punch, but the die hole is made progressively larger as the metal workpiece thickness increases in order to punch a clean hole without laps or tears As a result, a hole punched in a thicker workpiece is tapered rather than straight because it has the punch diameter at the punch side and the larger die hole diameter at the support side. In addition, the workpiece is frequently permanently bent during the punching operation by a moment generated in the workpiece as a result of the difference in diameters between the punch and the die hole. The bending effect is most pronounced when the workpiece width (distance from the edge of the workpiece to the hole) is small compared to the hole diameter, or when the workpiece is made of a soft material.
If a less distorted, e.g., straighter, punched hole be desired a two-diameter punch and shave procedure is commonly used. That procedure utilizes two punches of different diameters. The first punch and the die hole used therewith have normal diameters for the hole being punched, and produce the tapered hole described above. Because the second punch is used to remove or "shave" relatively little material, the second punch has a larger diameter than the diameter of the first punch, and the diameters of the second punch and the die hole used therewith are closer to each other than for the first punch and die hole. While that two diameter punch and shave procedure produces straighter holes, the workpiece bending described above still occurs.
In conventional cross hole punching operations, the die hole diameter is greater than the punch diameter by some percentage of the material thickness. The actual percentage varies depending upon the material and its hardness, and is typically about 10% of the material thickness For example, for a material thickness of about 0.150 inch, and a nominal desired cross hole of 0.200 inch, the punch diameter would be about 0.200 inch and the die hole diameter would be about 0.230 inch. For a shaving operation, the die hole diameter would exceed the punch diameter by only a few thousandths of an inch, e.g., the punch diameter would be about 0.200 inch and the die hole diameter about 0.205 inch.
There is, therefore, a need for a method and apparatus for producing low distortion holes at low cost, without distorting the workpiece, using punching as opposed to drilling.